1. The Field of the Invention
The present invention relates to a cook top comprising a colorless glass ceramic plate with a cooking surface on a top side thereof, which is provided with an opaque coating on an underside thereof, but which has at least one window area. The opaque coating is omitted in the at least one window area, which is provided with a special coating on the underside of the glass ceramic plate.
2. Related Art
Modern kitchens typically have a cook top with a glass ceramic plate providing a cooking surface. In order to prevent observation of parts of the cooking apparatus, such as the heating units, circuit boards, etc, which are under the cook top, which is undesirable for aesthetic reasons, typically the glass, from which the precursor glass plate to be ceramicized, also called the green glass plate, is made, is colored in the melt. Because of that it appears dark to black, when observed from above, which guarantees the required opacity.
When the glass is not colored in the melt, i.e. a transparent glass plate is made and it is ceramicized, predominantly high quartz-mixed crystals are formed, so that the glass ceramic plate is also transparent for visible light. In order to attain the required opacity the transparent glass ceramic plate is given an opaque coating on its underside in a known way, if necessary with an associated top decoration.
Modern glass ceramic cook tops have a window, in which the operation states or parameters, e.g. the selected heating stage and/or the residual heat in the cooking zones, are displayed by illuminating means. Also functional areas, with which the cooking zones can be turned on and off or the heating stage can be adjusted by touch, are usually found in the vicinity of the window or in the window itself. Touch is converted into an electrically signal by a so-called “touch sensor” under the glass ceramic.
The windows, under which touch sensors are arranged in addition to the illuminating means, are called “display or window areas” in the following disclosure.
So that the illuminating means (e.g. incandescent lamps, LEDs, LCDs or OLEDs) arranged under the glass ceramic plate behind the window area are easily read under the usual conditions in the kitchen, the glass ceramic plate must be sufficiently transparent in the region above the illuminating means, i.e. in the window area, for the wavelengths of the emitted light. This requirement prohibits an opaque topside decoration or opaque underside coating in the display areas.
Hence light impermeable opaque underside coatings as well as more economical topside decorations are left off of the area in which the display window is located, especially in glass ceramic-cooking surfaces made from colorless glass ceramic material.
The omission of a coating on a glass ceramic plate providing a cooking surface above a display device was described for the first time in 1991 in EP 0 438 656. The aim was to display the operating state of a cooking zone by means of an illuminated symbol (circle, line, or ring). The disclosure leaves open whether the coating is omitted from the top or bottom side of the glass ceramic.
EP 1 435 759 B1 describes the omission of a glass-based underside coating from a colorless glass ceramic plate providing a cooking surface. Similarly this possibility is mentioned in EP 1 267 593 B1 and in JP(A)2003 338 359.
In JP(A)2003 338 360 a two-layer glass-based opaque coating is provided, which comprises a first layer of lustrous paint and a second layer of an organic e.g. silicone-based coating. Both coatings must be omitted in the vicinity of the window area above the display device.
JP(A)2005 090 906 and WO/03 098 115 A1 disclose omitting a sputtered underside coating in order to provide a window area and improving the display quality by increasing the upper and underside antireflection layer.
According to DE 299 02 875 U1 the display device around a hot zone is formed by omitting parts of an underside, light impermeable coating on a dark colored glass ceramic plate providing a cooking surface so that e.g. only points or letters are observable instead of an illuminated ring.
While—like in the last named reference—with a colored glass ceramic plate providing a cooking surface, such as that marketed under the trademark CERAN SUPREMA®, no significant aesthetic properties are lost by removing the coating on its underside, with a colorless glass ceramic plate, especially when the display device is turned off, the interior of the cooking range can be seen through the window, because there the underside coating is removed. The observation of parts in the interior of the cooking range, for example circuit boards, is undesirable for aesthetic reasons.
A two-layer structure for the underside coating is thus disclosed in JP(A)2003 086 337, in which the first layer comprises an effect pigment, which is permeable for the light from the display device and the second layer comprises a light impermeable layer of a black pigment, e.g. Fe—Cr—Co Spinel. When only the second layer is omitted above the display device, the activated display device is clearly visible through the effect pigment layer, while the interior of the cooking range is sufficiently concealed by the effect pigment when the display device is turned off. The same device is disclosed in JP(A)2003 297 540.
However the underside coating of the display area with an effect pigment layer has the disadvantage that the effect pigment layer can easily crack or scratch because of the absence of the black covering layer during transport or assembly of the cooking range. Furthermore the light of illuminating means is noticeably scattered by the effect pigments, because the pigments are a few micrometers in size so that the display device does not have sharp edges and thus appears to be blurry. From FIG. 4 it is apparent that the fraction of the scattered light from this sort of window, like those in marketed cook tops, can amount to up to 30%. Because of the great scattering the illuminating devices are not clearly detectable. Display windows with effect pigment layers can thus be a safety hazard because their poorly readable displayed values in cases in which display devices that indicate the heating stage are arranged under them.
The light of the display device can even be scattered in the case of colored glass ceramic plates, e.g. by sufficiently large micro-crystallites in milky colored glass ceramic plates, which is disclosed in US 2005/0224491.
Often colored glass ceramic plates have nubs or bumps on their underside, which can lead to distortion of the display device. For that reason a smooth underside is produced in the display area according to the above-mentioned reference, as is also the case in DE 041 04 983 C1, to which a colorless silicone layer is applied.
According to the best known solution for providing a cook top comprising a transparent colorless glass ceramic plate with a window for a display device, which only weakly scatters light of the display device and prevents viewing the cooking range interior under the cook top, the underside coating of the glass ceramic comprises a noble metal preparation in the display area under the window.
Since the noble metal coating contains no pigments, it scatters very little visible light (wavelengths 400 to 750 nm). The scattering of the visible light amounts to 1 to 3% near this sort of window, so that the illuminating means under the window is comparatively clear. Curves a and b of FIG. 5 show, for example, the amount and behavior of the scattering for two different commercially obtainable glass ceramic plates providing cooking surfaces. FIG. 6a shows a segmented display in the window of a commercial glass ceramic plate providing a cooking surface. At the same time the ability to observe the interior of the cooking unit has been sufficiently greatly limited by the primarily black-brown noble metal coating, which has a transmission for visible light of about 0 to 45% (curve a, FIG. 7).
The noble metal coating has a very small electrical conductivity. Its surface resistance is around 1 MΩ/□. Because of that not only illuminating means but also touch sensors, which operate on a capacitive principle, can be arranged under the noble metal coating and operated. Touch sensors, which operate according to a capacitive principle, are currently used in current cooking ranges. It is common to arrange the sensors in the vicinity of the window as well as the illuminating means and to form the display area in this way.
The mechanical strength of the noble metal coating is sufficient for transport, assembly, and operation of the cook top.
The transmission curve a in FIG. 7 shows that known noble metal coatings are nearly impermeable for violet light (only about 0.6% transmission at 400 nm), while they transmit red light well (about 20 to 45% transmission at 700-750 nm).
Since the known noble metal coatings nearly completely block light of wavelengths 400 to 450 nm, they have the disadvantage that they are unsuitable for violet or dark blue illuminating devices. Because the known noble metal coatings have greatly varying transmittances for visible light, which has wavelengths from 400 to 750 nm, they are hardly suitable for multi-color displays, i.e. displays that use many different color shapes for display of information. When the known noble metal coatings are used to form the window for a multi-color display, the display may appear imbalanced in regarding to the intensities of the individual colors because of the wavelength dependent varying transmission of these noble metal coatings. However multi-color displays or also elegant dark blue displays will be more frequently used in glass ceramic plates providing cooking surfaces in the future because of the increasing diversity of electronic functions.
Because of the reduced transmission of the currently used noble metal coatings in the concerned wavelength range of 400 to 450 nm information displayed in violet or dark blue cannot be observed or only observed with reduced light intensity, while information displayed in red (700 to 750 nm) is observed to be extremely intense and correspondingly bright. The difference in the intensities of the designated colors, especially the intense red, for which the human eye is most sensitive, could produce an unpleasant impression during operation of the cook top.
The color shades could either be produced by light sources, which emit their respective wavelengths or elegantly by additive color mixing of the primary spectral colors red, green, and blue. The term “color shades” means the entire color palette from violet (400 nm) through blue (450 nm), green (550 nm), yellow (600 nm), orange (650 nm) until at red (700 nm) and dark red (750 nm) according to Hollemann-Wilberg, Lehrbuch der Anorganishcen Chemie (Textbook of Inorganic Chemistry), 91-100. Edition, Walter de Gruyter, Editor, Berlin, 1985, p. 103). The currently used noble metal coatings impair the display of colored information in both processes.
An additional disadvantage of the window of the commercial glass ceramic plates providing a cooking surface is that the display has very small dark areas or spots (FIG. 6a, FIG. 10). Because of that the illuminated segments of the e.g. seven part display appear to be spotted. It is very desirable that an illuminating means arranged under the glass ceramic plate uniformly illuminated the window area, especially in the case of expensive high quality cooking surfaces.
In summary the known solution for coating a window area or display area provided on a colored, transparent, smooth-surfaced (especially both sides) glass ceramic plate does not provide satisfactory uniform light transmission in the visible range and does not provide acceptable clarity of displayed information produced by a display device below the window area.